1. Field of the Invention
The invention relates generally to the field of metallurgy and, more particularly, to ultra-high strength stainless steel with enhanced toughness which is suitable for applications such as aircraft landing gear, high strength bolts, airframe parts and the like.
2. Description of Related Art
For many years, low alloy steels such as 300M plated with cadmium have been used in the manufacture of aircraft landing gear. Cadmium is used to improve corrosion resistance of the underlying high strength steel but comes at a cost of environmental problems associated with cadmium. There is, thus, a strong motivation to develop an alloy which has the needed strength and toughness for landing gear applications but which is more environmentally friendly. It is known that 300M has a very good combination of strength (280 to 305 ksi), toughness, fatigue strength and good ductility but, as noted, is not resistant to corrosion. Accordingly, there is a need for an ultra-high strength stainless steel with good toughness. Stainless steels of this type have received little attention since the work of Asayama in the early 1970s, see Asayama articles “Notch Toughness Characteristics of High Strength Maraging Stainless Steel”, Jpn. Inst. Of Metals Journal, 1976, 40, No. 5, p. 533; “The Effect of Aging on the Notch Toughness of High Strength Maraging Stainless Steels”, Jpn. Inst. Of Metals Journal, 1977, 41, No. 10, p. 973; and “Study on Aging Embrittlement of High Strength Maraging Stainless Steels”, Nippon Kinzokee Gakkaishi, 1978, 42, No. 7, p. 649.